A primary concern in using an optical fiber as a light-conducting means is the efficient coupling of light into the end of the fiber. Since the planar end of an optical fiber has a narrow light-acceptance angle, only a portion of the light directed at the fiber end actually enters the fiber core. This problem is compounded when light from a source of divergent rays, such as a semiconductor laser, is coupled into a small diameter core of a single mode fiber. For example, a typical semiconductor laser has a beam divergence angle of about 40.degree. and a single mode fiber has a light-acceptance angle of about 6.degree..
In addition to this inherent problem, laser-to-fiber couplings are also characterized by low tolerances. For example, when a laser and fiber are axially aligned along the optical axis of the fiber, the light emanating from a 4 micrometer (.mu.m) wide lasing spot is directed at the end of a 10 .mu.m diameter fiber core. Under these circumstances even submicron misalignment along the orthogonal axes can greatly reduce the percentage of power detected at a second end of the fiber.
Attempts to attach small lenses to the end of the fiber or to impart various shapes to the fiber end, by etching, grinding or melting, can increase the light-acceptance angle. These solutions, however, are characterized by poor reproducibility and damage to the fiber end. Finally, although these fiber-end lenses serve to increase the light acceptance angle, the device-to-fiber alignment must still be within a tolerance of 1 .mu.m or less.
Higher tolerance systems, i.e. systems wherein greater leeway in the orthogonal directions is acceptable, involve separate optical components interposed between the laser and fiber. A prior art system using three lenses interposed between the laser and fiber has provided tolerances on the order of several micrometers by producing a magnified image of the laser. However, efforts have been directed at reducing the number of components in a package since each additional component reduces the likelihood of an accurately aligned package. Two-lens, high tolerance systems which typically use a spherical lens adjacent the laser are characterized by spherical aberrations because the outermost portions of the spherical lens are utilized to collimate the highly divergent rays. Another two-lens system is the confocal system wherein the two lenses are fixed such that their principal axes are separated by the sum of their focal lengths. Although this arrangement provides higher tolerances for the positioning of the lenses, the final positioning of the fiber must be within a few micrometers.
Therefore, a high tolerance, high efficiency optical package using a minimum of optical components and a method for its assembly has been sought.